The Allen-Bradley 1747-L551 is the SLC 5/05 modular processor with 16K words of user memory and built-in Ethernet plus RS-232 communications. As an independent third-party supplier of obsolete and hard-to-find automation parts, BXLY PLC supplies the 1747-L551 and the wider SLC 500 processor family for plants still running this discontinued platform. This guide covers its specifications, compatible replacements, common failure points and how to request a quote.
Quick Specifications
| Attribute | Detail |
|---|---|
| Manufacturer | Allen-Bradley (Rockwell Automation) |
| Series | SLC 500 — modular; SLC 5/05 |
| Type | Processor / CPU module |
| Catalogue number | 1747-L551 |
| User memory | 16K words total (12K words program/data + 4K words data-only) |
| Communication ports | 1 × RS-232 (DF1 full/half duplex, DH-485) + built-in Ethernet |
| Ethernet speed | Series C: 10/100 Mbps; earlier Series A/B: 10 Mbps *(confirm the series marking on your unit ) |
| I/O capacity | Up to 4096 inputs + 4096 outputs absolute maximum |
| Chassis / slots | SLC 500 modular chassis, up to 30 local slots |
| Memory backup | 1747-BA lithium battery (battery-backed RAM) |
| Engineering software | RSLogix 500 |
| Status | Discontinued / End-of-Life — no longer manufactured; original manufacturer no longer supplies |
| Compatible alternates | 1747-L552 (32K) / 1747-L553 (64K) |
| Migration target | CompactLogix 5380 |
What the 1747-L551 Does
The 1747-L551 is the processor module of an SLC 500 modular system. It executes the ladder program, scans local and remote I/O, and hosts the communication channels that link the controller to HMIs, drives and plant networks. Its defining feature within the SLC 500 range is that it was the first SLC processor to offer a built-in Ethernet port, letting it talk to PanelView terminals and PCs over Ethernet without adding a separate communication module.
Its 16K words of user memory place it at the entry point of the SLC 5/05 line — above the SLC 5/03 and 5/04 processors, but below the 1747-L552 (32K) and 1747-L553 (64K). A single RS-232 port supports DF1 and DH-485, so a serial HMI or a legacy programming connection can be served alongside the Ethernet channel.
The 1747-L551 mounts in an SLC 500 modular chassis and works with the full 1746 digital and analog I/O family — commonly up to 30 local slots, with remote I/O added by network. Because the SLC 500 platform is now End-of-Life, the practical decision for most sites is whether to drop in a same-family spare or plan a CompactLogix migration.
Compatible & Alternative Modules
| Alternative | Family | User memory | Key difference vs. L551 |
|---|---|---|---|
| 1747-L552 SLC 5/05 32K | SLC 5/05 | 32 K words | Same Ethernet + RS-232 layout, double the user memory — closest like-for-like upgrade. |
| 1747-L553 SLC 5/05 64K | SLC 5/05 | 64 K words | Same layout, quadruple the memory of the L551 — top of the 5/05 range. |
| 1747-L541 / L542 / L543 | SLC 5/04 | 16K / 32K / 64K | Uses DH+ networking instead of Ethernet; not a straight communication substitute. |
| 1747-L531 / L532 | SLC 5/03 | 8K / 16K | Lower-tier SLC processor with DH-485 serial networking only. |
| CompactLogix 5069-L3xx | CompactLogix 5380 | n/a | Rockwell's recommended migration target — new programming environment and I/O platform, not a drop-in. |
Compatibility notes
- The 1747-L552 and 1747-L553 are the natural in-kind upgrades when you simply need more memory or a direct spare in the same 5/05 family — the Ethernet/RS-232 interface and programming method are shared.
- Memory is battery-backed RAM; a discharged 1747-BA battery means the program is lost on power-down, so a replacement CPU must be re-downloaded from your project file.
- Moving to CompactLogix 5380 is a migration project, not a one-for-one swap: it changes the programming software, the I/O platform and the wiring.
Common Failure Points & Replacement Signs
- Low / dead backup battery (BATT or LOW BAT indicator). The 1747-BA battery preserves the program in battery-backed RAM. A discharged battery lights the front-panel indicator and, if power is cycled, the program can be lost entirely — the most common reason a working SLC 5/05 suddenly "loses its program."
- Ethernet communication failure. A flashing fault LED with a solid Ethernet LED is a classic no-comms pattern; the on-board Ethernet port can fail outright on high-hour units even though the processor still runs.
- Memory loss during hot-swap. Removing or inserting the processor or modules while the rack is powered can corrupt or erase memory. Always power down before swapping hardware.
- Power-section damage. The processor's power/regulator section is a frequent repair item — symptoms include a unit that will not power up, browns out, or resets randomly.
- Capacitor ageing and erratic faults. On units well past their service life, ageing capacitors and connectors produce intermittent faults, flickering LEDs and unreliable serial/Ethernet connections — a signal to replace rather than keep repairing.
Availability & Lead Time
BXLY PLC supplies the 1747-L551 SLC 5/05 16K controller and the SLC 500 family as an independent third-party supplier, across several condition grades — new old stock / factory surplus, refurbished and tested pulls, and used / as-removed units with an honest condition report. We do not publish prices on this page; availability, condition and lead time are confirmed per enquiry.
Request a Quote → with your required condition (new / refurbished / used), quantity and destination, and we will confirm live stock and lead time. A warranty applies to tested stock (terms confirmed on your quotation). Because these processors are battery-backed, we recommend ordering a fresh 1747-BA battery with your replacement CPU and re-downloading the program at commissioning.
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*BXLY PLC is an independent third-party supplier of obsolete and hard-to-find automation components and is not affiliated with, authorized by, or an authorized distributor of Rockwell Automation / Allen-Bradley. All trademarks are the property of their respective owners. Specifications are compiled from manufacturer documentation and are provided for identification purposes; always confirm the exact revision against your project.*